In fields involving microelectronic devices, sensors, and optical elements, the development of devices that are small relative to the state of the art, controllable, and conveniently and relatively inexpensively reproduced with a relatively low failure rate is important.
There is currently great interest in the fabrication of optical devices based on organic materials that display electroluminescence (Burrows, et al., Current Opinion in Solid State and Materials Science, 1997, 2, 236; Baigent, et al., Synth. Meth., 1994, 67, 3). These devices could find application in a variety of settings, including flat panel optical displays. Electroluminescence, including in some cases organic electroluminescent materials, is described by Yam, “Plastics Get Wired”, Scientific American, July, 1995, 83-87; Kijima, et al., “RGB Luminescence from Passive-Matrix Organic LED's”, IEEE Transactions on Electron Devices, 44, 8, August, 1997; Shen, et al., “Three-Color, Tunable Organic Light-Emitting Devices”, Science, 276, Jun. 27, 1997; and Burrows, et al., “Achieving Full-Color Organic Light-Emitting Devices for Lightweight, Flat-Panel Displays”, IEEE Transactions on Electron Devices, 44, 8, August, 1997. A variety of materials, including electroluminescent materials, have been deposited on surfaces at small feature size using a variety of techniques including laser ablation, photolithography, the use of shadow masks, and other techniques.
Burger, et al., in “High-Resolution Shadow Mask Patterning in Deep Holes and its Application to an Electrical Wafer Feed-Through”, Sensors and Actuators, A 54 (1996) 669-673, describe electron-beam evaporation of metals through a shadow mask.
Wang, et al. in “identification of a blue photoluminescent composite material from a combinatorial library” Science, 279, Mar. 13, 1998, 1712-1714, describe a quaternary combinatorial masking strategy used in conjunction with photolithography to generate compositionally diverse thin-film phosphor libraries.
Noach, et al., in “Microfabrication of an Electroluminescent Polymer Light Emitting Diode Pixel Array”, Appl. Phys. Lett., 69 (24), Dec. 9, 1996, describe a technique for the fabrication of a light emitting diode array based on conjugated electroluminescent polymers sandwiched between appropriate electrodes. The method is based upon direct photoablation with an excimer laser.
Renak, et al., in “Microlithographic Process for Patterning Conjugated Emissive Polymers”, Adv. Mater., 1997, 9, 5, 392-394, describe a microlithographic process for patterning electroluminescent poly(p-phenylenevinylene) (PPV), and show preliminary results from pixel-like LED arrays prepared by this technique. Photoacid generators, molecules that generate acids upon photolysis, were used. A photoacid generator admixed with a PPV precursor is spin-cast onto a substrate, according to the technique, followed by irradiation through a mask to promote formation of triflic acid, in turn generating PPV. Development in chloroform washes away un-reacted precursor, leaving patterned PPV.
Granstrom, et al., in “Micrometer- and Nanometer-Sized Polymeric Light-Emitting Diodes”, Science, 267, Mar. 10, 1995, describe a technique for the fabrication of light-emitting diodes by polymerizing doped and conducting polymers electrochemically in randomly-distributed pores of commercially-available microfiltration membranes. Polycarbonate membranes were used. Electroluminescent polymer was spin-coated from xylene solution on top of a microfiltration membrane-contact structure.
While the above-described and other techniques may be advantageous in many circumstances, many require relatively sophisticated and expensive apparatus and/or require excessive numbers of steps or potentially destructive chemical techniques. For example, laser ablation is relatively slow and requires relatively complex apparatus. Also, it is limited in that pixels are defined by patterning cathode material rather than the electroluminescent material itself, and therefore the technique is not well-suited to creation of a multi-colored display. Photolithography is generally faster than laser ablation and therefore has greater potential for high-volume production of useful devices. However, wet chemical etching generally is required to define pixels using photolithography, which has a deleterious effect on electroluminescent efficiency. Lidzey, et al. (Synth. Meth., 1996, 82, 141) report that the efficiency of an electroluminescent device was decreased by 60 percent upon one photolithographic step involving a wet chemical etch. Devices also can be degraded by exposure to atmospheric water and oxygen, inevitable during photolithography. Encapsulation has been used as a technique to avoid degradation during photolithography (Tian, et al., Appl. Phys. Lett., 1997, 71, 3197). Encapsulation, however, is problematic, according to Tian, et al., who report that it can lead to shorting of devices. Encapsulation also adds an extra step to the fabrication process. Shadow masking techniques that are known typically involve the use of machined-metal masks, in which the size of pixels are generally considered too large for high-resolution displays. In many of the above techniques, it is not possible to fabricate displays on non-planar surfaces.
Hebner, et al. in “Ink-Jet Printing of Doped Polymers for Organic Light Emitting Devices”, Appl. Phys. Let. 72, 5, Feb. 2, 1998, describe patterning luminescent-doped polymer films using ink-jet printing.
International patent publication WO 97/33737 (International patent application serial number PCT/US97/04005) of Kim, et al., entitled “Method of Forming Articles and Patterning Surfaces Via Capillary Micromolding”, describes techniques for applying a variety of species to surfaces according to predetermined patterns. An elastomeric article having a contoured surface including a plurality of protrusions and intervening indentations is positioned against a substrate surface so that the outward-facing surfaces of the protrusions contact the substrate surface. In this way a plurality of channels are created, each defined by the surface of an indentation and a portion of the substrate surface in register with the indentation. A fluid carrier or precursor is introduced into the channels, an agent is deposited at regions of the substrate surface corresponding to the channels, and the article is removed from the surface. Patterned chemical reactions, precipitation, polymerization, and the like can take place at the substrate surface in this manner. Alternatively, an agent can be positioned in the indentations and the contoured surface brought into contact with a surface to be modified. Also described in Kim, et. al. is a flexible polymeric mask used to pattern deposition of material on a surface.
Rogers, et al., (Appl. Phys. Lett. 7, 70, 1997) describe a technique for forming a photomask on the exterior surface of an optical fiber. An elastomeric article, having a contoured surface including protrusions and intervening indentations, is used to apply a chemical species to the exterior surface of an optical fiber according to the pattern of the protrusions. Specifically, the axis of the optical fiber is positioned perpendicularly to the protrusions and is rolled across the protrusions, and rings of the chemical species are transferred from the protrusions to the outer surface of the fiber in this way. The chemical species can facilitate creation of a photomask by serving as a metal deposition catalyst.
Several physical masking techniques are known for application of specific materials to specific portions of a surface.
Flexible masks are known for use in selective exposure of photoresist in a pattern corresponding to the pattern of the mask. U.S. Pat. No. 4,735,890 (Nakane) describes a photomask for photolithographic fine patterning of a photoresist film. A thin film of a polymeric material having elasticity is brought into intimate contact with a photoresist film. Selective exposure of the photoresist through the photomask allows desired patterning of photoresist. Other “contact photolithography” techniques, involving contact between a mask and a photoresist coated substrate, are described in U.S. Pat. No. 5,147,763 (Kamitakahara) and U.S. Pat. No. 5,160,959 (Everett) and U.S. Pat. No. 4,810,621 (Akkapeddi).
U.S. Pat. No. 5,259,926 (Kuwabara) describes a technique for thin-film patterning. A thin film is provided on a substrate and a mask, having a desired pattern, is formed on the thin film by forming a layer of an organic resin on the thin film and forming the organic resin layer in a desired pattern by a mechanical forming member. An exposed portion of the thin film then is removed by etching.
U.S. Pat. No. 4,518,636 (Richards) describes a technique for selective metal plating of a component. Upper and lower faces of the component are contacted with upper and lower masks, respectively, so that the lower mask exposes a part of the component to be plated. The part is positioned over a plating tank and selective plating takes place. The upper mask can be a deformable elastic polymeric material, and the lower mask is a more rigid rubber or plastic material.
U.S. Pat. No. 5,480,530 (Zejda) describes an elastomeric mask for covering the outer marginal area of a disk-shaped substrate surface during a coating process. The substrate can be a compact disk, and the mask is of an annular shape with a circular opening into which the disk is placed. An inner, substrate holder, inserted into a central hole of the disk, also made of elastomeric material.
U.S. Pat. No. 5,691,018 (Kelly) describes a flexible elastomeric mask for protecting apparatus used to mount a work piece to be subjected to thermal spray coating.
U.S. Pat. No. 5,705,043 (Zwerner) describes apparatus for selectively electrolytically plating defined regions of a continuously moving conductive work piece. Elastomeric sealing plates are provided including openings that define areas of work pieces to be plated.
Described above are several techniques for creation of a variety of materials, including electroluminescent materials, that in many cases are complicated and expensive. Also described above are several masking techniques, many of which do not recognize application to very small-feature, very high-resolution techniques. Accordingly, one object of the present invention is to provide high-resolution optical devices that can be multi-color and can display electroluminescence. Another object of the invention is to provide apparatus and techniques for forming such displays. It is another object of the invention to provide improved apparatus and techniques for forming a variety of patterns of a variety of materials on a variety of surfaces at high resolution.